Seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) of the strata underlying the land surface or seafloor. Among other things, seismic data acquisition involves the generation of acoustic waves, the collection of reflected/refracted versions of those acoustic waves, and processing the collected seismic data to generate the image. This image does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing an improved image of the subsurface for avoiding drilling a dry well is an ongoing process in the field of seismic surveying.
Seismic data acquisition is typically conducted in a land or marine environment. A configuration for achieving land seismic data is illustrated in FIG. 1. FIG. 1 shows a system 100 that includes plural receivers 102 positioned over an area 104 of a subsurface to be explored and in contact with, or below the surface 106 of, the ground. A number of dedicated seismic sources 108 are also placed on the surface 106 in an area 110, in a vicinity of the area 104 of the receivers 102. Note that a dedicated seismic source is defined as a device built by man with the main purpose of generating seismic waves to be used for a seismic survey. Alternatively, dedicated seismic sources 108 may be buried under surface 106. A central recording device 112 is connected to the plurality of receivers 102 and placed, for example, in a station/truck 114. Each dedicated seismic source 108 can be composed of a variable number of vibrators, typically between one and five, and can include a local controller 116. A central controller 118 can be provided to coordinate the shooting times of sources 108. A global positioning system (GPS) 120 can be used to time-correlate shooting of the dedicated seismic sources 108 and the recordings of the receivers 102.
A configuration for achieving marine seismic data is illustrated in FIG. 2. A marine seismic data acquisition system 200 includes a survey vessel 202 towing a plurality of streamers 204 (one shown) that may extend over kilometers behind the vessel. One or more source arrays 206 may also be towed by the survey vessel 202 or another survey vessel (not shown) for generating seismic waves 208. Conventionally, the source arrays 206 are placed in front of the streamers 204, considering a traveling direction of the survey vessel 202. The seismic waves 208 generated by source arrays 206 propagate downward and penetrate seafloor 210, eventually being reflected by a reflecting structure 212, 214, 216, or 218 at an interface between different layers of the subsurface, back to the surface 219. The reflected seismic waves 220 propagate upward and are detected by detectors 222 provided on the streamers 204. This process is generally referred to as “shooting” a particular seafloor 210 area. A similar setup may be used for an ocean bottom node acquisition system, in which the seismic detectors are directly placed on the ocean bottom 210 and vessel 202 only tows seismic source 206 for generating the seismic waves 220.
A typical problem encountered with all of these seismic acquisition systems is the presence of surface waves. FIG. 3 schematically illustrates a seismic acquisition system 300 having a source 306 and a seismic detector 322 located on ground surface 310 (it can be earth's surface or ocean bottom). Source 306 emits seismic waves. Part of the energy propagates as body waves 319, downward, toward various interfaces 314 and layers of the earth while part of the energy propagates as surface waves 312, at the air-ground interface (for land surveys) or water-ocean bottom interface (for marine surveys).
The surface waves carry a lot of the energy generated by the source and they propagate without radiating into the Earth, i.e., parallel to the Earth's surface. These energies are typically considered to make up the coherent noise in seismic data. The surface noise may include one or more of Rayleigh waves, Lamb waves, Love waves or Scholte waves.
Because the surface waves propagate in the shallow portion of the Earth, they depend on the elastic properties of the superficial Earth, which is known as the near-surface region. Thus, if these waves can be separated from the seismic data and analyzed, they can contribute to the knowledge of the elastic properties in the near-surface. Alternatively, if the objective is to remove them from the acquired seismic data, in order to design accurate filters for achieving this goal, the properties of the surface waves need to be known.
The surface waves are dispersive, i.e., their propagation velocities vary with their frequency. In other words, at low frequencies, the surface waves have a long wavelength and can sample deep layers of the subsurface while at high frequencies, the surface waves have a shorter wavelength and therefore they can sample shallower layers of the subsurface. The term “dispersion curve” describes the behavior of the surface wave as a function of its frequency (or its period, or pulsation). It can be any combination of group velocity/phase velocity/arrival time vs frequency/period/pulsation.
However, the recorded seismic data that includes the surface waves includes many propagation velocities that correspond to a same frequency of a given surface wave, i.e., the data is ambiguous and it needs to be disambiguated.
As the present geophysical acquisitions patterns become ever denser, both in number of sources and receivers, thus giving high redundancy subsurface characterization, there is a need to obtain the propagation velocities of the surface waves as a function of their frequency (the dispersion curve) that take advantage of this high redundancy.